How Wi-Fi routers sense movement through walls

HostWe all have that one little box in the house, usually tucked behind a plant or sitting on a shelf, that keeps our phones and laptops connected to the world. We think of it as a tool for the internet, but lately, people are figuring out that those same signals are bouncing off our bodies to map out exactly where we're in the room. It sounds like something out of a spy movie, but there are no lenses involved. How can a plastic box with no eyes tell if I'm sitting on the couch or walking to the kitchen?

GuestIt helps to think of your router not as a data machine, but as a light bulb that never stops flashing. It's filling your home with waves of energy. These waves go through walls, but when they hit a person, they don't just pass through easily. They bounce, they bend, and they scatter. If you move, you change the way those waves look when they get back to the router or your phone. It's almost like walking through a room full of invisible hanging strings. When you move, you tug on those strings, and the box on the wall feels the vibration. Scientists have realized that the mess created by those bounces isn't just noise. It's actually a very detailed map of what's happening in the room.

HostSo it isn't taking a photo. It's more like it's feeling the shape of the room?

GuestThat's a good way to put it. Every time a signal travels from the router to your phone, it takes a whole lot of different paths. Some go straight there, but others hit the floor, the ceiling, and you. For a long time, engineers worked really hard to get rid of the reflections because they thought it made the internet slower. But then they saw that the reflections were telling a story. If the signal looks a certain way, it means there's a solid object in the middle of the floor. If that signal starts shifting in a steady rhythm, it can even mean that object is breathing. By looking at how the waves get bunched up or stretched out, the software can figure out what's a human and what's just a chair.

HostWait, it can pick up something as small as a chest moving up and down? That feels a bit much. I mean, surely a wall would block something that tiny.

GuestWell, here is the thing about the walls in most of our homes. To a Wi-Fi wave, drywall and wood are basically clear. They're like glass. The waves pass right through them, hit you, and bounce back through the wall again. The router is sending out these check-in pulses hundreds of times every single second. Because it's doing this so fast, it can see very small changes. When you breathe, your chest only moves a tiny bit, but that's enough to change the timing of the wave by a fraction of a second. The computer looks at those shifts and says, okay, that's not a fan blowing or a curtain moving in the wind. That's a human heart beating or a pair of lungs filling up. It's so sensitive that it can track your sleep quality from a different room.

HostBut my house is full of stuff. There are dogs, there are robot vacuums, there's a dishwasher running. How does it not get confused by all that other movement?

GuestThat was the biggest hurdle for a long time. The air is very crowded. To fix this, we started teaching the software to recognize patterns. A human has a very specific way of moving. When we walk, our arms swing and our hips move in a certain rhythm. A dog moves on four legs and has a totally different bounce. A robot vacuum is just a flat disc moving at a steady speed. We showed computers thousands of examples of what these different movements look like in the form of radio waves. Now, the software can look at the jagged lines of the signal and pick out the human signature from all the other clutter. It's not seeing your face, but it's seeing the unique way your body moves through space.

HostI can see how that would be great for helping an older person who lives alone. If they fall, the house would know and could call for help. But what about the privacy side? If the waves go through walls, does that mean my neighbor's router knows when I'm in the shower?

GuestTechnically, the signals from your neighbor's house are probably in your home right now. They're bathing your living room as we speak. But it's not quite as simple as someone just opening an app to watch you. To make sense of the data, the software has to be tuned to the specific layout of a room. It needs to know where the big heavy things like fridges and sofas are so it can ignore them. Also, the data doesn't look like a video. It looks like a bunch of messy graphs. Without the right key to read those lines, it's just gibberish. That said, the tech is getting much better. Some new systems can now guess where your arms and legs are, creating a sort of stick figure version of you.

HostIt's a lot to take in. We think of Wi-Fi as this invisible pipe for our movies and mail, but it's actually a live map of our private lives.

GuestThe air in our homes is becoming a part of the computer itself. We're moving toward a world where the house just knows where you are. It can turn off the lights when a room is empty or aim the heater right at you as you sit down to read. The waves are always there, bouncing off our skin and carrying the story of every move we make to any device that knows how to listen.

HostThat little plastic box behind the plant is doing a lot more than just loading our web pages.